1. Field of the Invention
The present invention relates to a solid-state electron beam generator.
2. Related Background Art
A known solid-state electron beam generators is shown in, for example, specification of U.S. Pat. No. 4,259,678. This known electron beam generator has a pn junction formed on a Si semiconductor substrate. A reverse voltage is applied to the pn junction so as to produce avalanche effect thereby generating electrons (referred to as "hot electrons" hereinunder) having energy level higher than that in a thermal equilibrium state. An electron beam is then emitted into vacuum by the kinetic energy of the hot electrons.
In this known electron beam generator, the proportion of number of hot electrons having energy levels higher than the vacuum energy level to the total number of hot electrons produced by the avalanche effect is rather small, so that only a small electric current is obtained.
Another type of known solid-state electron beam generator has, as disclosed in Japanese Patent Publication No. 30274/1979, a pn junction composed of a Al.sub.x Ga.sub.(1-x) P layer (0.ltoreq.x.ltoreq.1) which is formed on a GaP semiconductor substrate and a forward voltage is applied to the pn junction region thereby causing emission of electrons which have been injected from the n-type region into the p-type region.
This solid-state electron beam generator can provide a greater number of carriers than in the first-mentioned known electron beam generator disclosed in U.S. Pat. No. 4,259,678, but the efficiency of emission of electrons into vacuum is impractically low because of lack of any region for forming hot electrons. In addition, a GaP substrate in general tends to have crystalline defects such that it is rather difficult to form good pn junction region.
In advancement of the above-mentioned two types of known solid-state electron beam generators, a solid-state electron beam generator has been proposed in the specification of U.S. Pat. No. 3,119,947 in which an npn region is formed on a Si semiconductor substrate and a voltage is applied between both n-type regions thereby causing electrons to be emitted. This known electron beam generator employing an npn junction can increase the emission efficiency to an order of 10.sup.-4 which is much higher than 10.sup.-6 which is obtained in the first-mentioned known electron beam generator which employs a pn junction. This solid-state electron beam generator, however, is generally difficult to produce because the p-type region and the n-type region on the emission side have to be formed in an extremely small thickness on the order of several hundreds of angstroms and, in addition, with a high degree of uniformity in thickness. Thus, the solid-state electron beam generator of the third type cannot easily be put into practical use.